The present invention relates to a solid-state imaging device and a process for production thereof, the solid-state imaging device being characterized in that each pixel has a convex lens with an upwardly curved surface which is embedded in an inter-layer dielectric between a light-receiving portion and an on-chip lens.
The CCD solid-state imaging device should desirably have a reduced chip size and an increased number of pixels. Unfortunately, this object is not achieved simply by reducing the chip size while leaving the current pixel size as it is because such an attempt ends up with a reduction in the number of pixels and hence a reduction in resolution. Nor is the object achieved simply by increasing the number of pixels while leaving the current pixel size as it is because such an attempt ends up with an increase in chip size and hence an increase in production cost and a decrease in yields.
Therefore, for reduction in chip size and for increase in the number of pixels, it is essential to reduce the pixel size from the current one. If this object is achieved, it is possible to provide a small-sized CCD imaging device which keeps the current resolution unchanged or conversely to improve resolution while keeping the current chip size unchanged.
The problem with reduction in pixel size is that the amount of light incident on a unit pixel decreases and the light-receiving portion of a unit pixel becomes poor in sensitivity characteristics. Although the second difficulty can be overcome by improving the photo-electric conversion efficiency with a concomitant adverse effect of amplifying noise components, the result is a decrease in S/N ratio of image signals output from the CCD imaging device. In other words, it is not desirable to compensate for the loss of sensitivity characteristics resulting from reduction in pixel size only with improvement in photo-electric conversion efficiency, but it is desirable to improve as much as possible the condensing efficiency of each pixel, thereby preventing the decrease in S/N ratio.
An idea contrived from the above-mentioned standpoint is to form an on-chip lens (OCL) on the color filter formed on the light-receiving portion. This idea, however, is not practicable for a CCD imaging device having a pixel size smaller than 4×4 μm, because the efficiency of condensing light with an on-chip lens alone has almost approached the upper limit. A new technology to overpass the limit has been proposed in Japanese Patent Laid-open No. Hei 11-40787. This technology is concerned with an additional convex lens in filmy form of light-transmitting material which is formed in the layer between the on-chip lens and the light-receiving portion. This additional convex lens is designed to improve further the efficiency of condensing light. According to the disclosure, the convex lens is formed by the process which is explained below with reference to FIGS. 6A to 7C. This process is referred to as “conventional process 1”.
As shown in FIG. 6A, the process starts with fabrication of a silicon substrate 1 to form thereon the following components in the conventional way. A light-receiving portion 2, charge transfer portions 3-1 and 3-2, a gate portion (not shown) between the light-receiving portion 2 and the charge transfer portion 3-1, and a channel stopper (not shown) between the light-receiving portion 2 and the charge transfer portion 3-2. Transfer electrodes 5 are embedded in the insulating film 4 covering the charge transfer portions 3-1 and 3-2. On the insulating film 4 is formed a shielding film 6 of high-melting metal which has an opening above the light-receiving portion 2.
Then, a BPSG film 20 is formed on the shielding film 6 and the opening therein. The BPSG film 20 is planarized by reflowing as shown in FIG. 6B. On the planarized film 20 is formed a light-transmitting film 21a from silicon nitride (P—SiN) or silicon oxide (P—SiO2) by plasma CVD as shown in FIG. 6C.
The light-transmitting film 21a is coated with a resist. The resist film is patterned such that a region around the center of the light-receiving portion 2 remains. The patterned resist undergoes reflowing, so that it softens and forms a convex lens (resist pattern RP) having a prescribed curvature, as shown in FIG. 7A.
Etching is performed under the condition that the resist and the light-transmitting film have almost the same selectivity. Etching removes the resist, while leaving the light-transmitting film in the form of convex lens 21. The shape of the convex lens 21 conforms well to the shape of the resist pattern RP, as shown in FIG. 7B.
Subsequently, the convex lens 21 is embedded in a planarizing film 9. Finally, an on-chip color filter (OCCF) 10 and an on-chip lens (OCL) 11 are formed in the usual way, as shown in FIG. 7C.
There is another process (“conventional process 2”) in which the convex lens forming step in “conventional process 1” is modified as explained below.
FIGS. 8A to 9C are sectional views showing “conventional process 2”.
As shown in FIG. 8A, “conventional process 2” starts with fabrication of a silicon substrate 1 to form the following components thereon as in “conventional process 1”. A light-receiving portion 2, charge transfer portions 3-1 and 3-2, an insulating film 4, transfer electrodes 5, and a shielding film 6.
A PSG film or BPSG film 22 is formed on the shielding film 6 and the opening therein. The PSG film or BPSG film undergoes reflowing. According to “conventional process 2”, this reflowing does not achieve complete planarizing but forms a depression 22a above the light-receiving portion 2.
As shown in FIG. 8B, a light-transmitting film 23a of P—SiN or P—SiO2 is formed on the PSG film or BPSG film 22. On the light-transmitting film 23a is formed resist R which is subsequently planarized.
Etching is performed under the condition that the resist R and the light-transmitting film 23a have almost the same selectivity. Etching forms the light transmitting film 23b, with its surface planarized, as shown in FIG. 8C.
As shown in FIG. 9A, a light transmitting film 23c is formed on the planarized surface. Then, a resist is applied to the light transmitting film 23c, and the resist film is patterned such that a region around the center of the light-receiving portion 2 remains. This patterning is following by reflowing. In this way there is obtained a resist pattern RP in the form of convex lens having a prescribed curvature.
Etching is performed again under the condition that the resist and the light-transmitting film have almost the same selectivity. Etching removes the resist, while leaving the light-transmitting film in the form of convex lens 23. The shape of the convex lens 23 conforms well to the shape of the resist pattern RP, as shown in FIG. 9B.
Finally, a planarizing film 9 is formed and an OCCF 10 and an OCL 11 are formed in the usual way, as shown in FIG. 9C.
The CCD imaging element obtained by “conventional process 1” and “conventional process 2” functions in the following way as shown in FIG. 10. The OCL 11 converges to some extent the incident ray (the vertical incident ray L0 and the oblique incident ray L1 with respect to the light-receiving plane). Another convex lens 21 (or 23) under the OCL 11 further converges the converged incident light. The converged light reaches the light-receiving portion 2. Thus, the convex lens 21 (or 23) improves the efficiency of condensing incident rays. The convex lens 21 (or 23) is particularly effective in condensing the oblique incident ray L1 indicated by broken lines in FIG. 10. This improves the sensitivity of each pixel.
“Conventional process 1” and “conventional process 2” which are designed to form the convex lens 21 or 22 under the OCL 11 have the disadvantage that the curvature of the surface of the convex lens 21 or 23 depends on the curvature of the surface of the resist to be used as a mask during processing. In other words, the curvature of the resist surface should be large if the curvature of the lens surface is to be large, and the curvature of the resist surface should be small if the curvature of the lens surface is to be small. This makes it necessary to optimize the resist thickness so that the lens surface has a desired curvature.
Unfortunately, forming the convex lens by “conventional process 1” or “conventional process 2” poses a problem that the resist thickness tends to vary as it is reduced in proportion to the pixel size. Uneven resist coating implies that the thickness of the convex lens and the curvature of the lens surface vary in the same chip or in the same wafer. This in turn results in uneven sensitivity, that is, the level of output signal varies from one pixel to another.
A conventional practice to address this problem was to apply the resist somewhat thick enough to avoid coating variation in the step of forming the convex lens.
Thick coating means that the shape of the convex lens deviates from the optimal value in the case where the pixel size is small. In other words, the convex lens 21 has an excessively large curvature of lens surface relative to the pixel size, as shown in FIG. 11. The large curvature of the lens surface moves the focus upward from the light-receiving plane, as shown in FIG. 11, with the result that light diverges on the light-receiving plane and the amount of light received decreases. For the vertical light L0 indicated by solid lines in FIG. 11, the amount of light received decreases rather slightly.
However, the amount of light received decreases remarkably when the ratio of oblique light L1 (relative to OCL 11) increases. This occurs when the camera equipped with the CCD imaging element in question has its lens stopper opened more (so that the F value is small). As indicated by broken lines in FIG. 11, a large portion of the light passing through the convex lens strikes the shielding film 6 outside the light-receiving plane. This results in a considerable decrease in pixel sensitivity.
Moreover, the light incident aslant on the opening 6a of the shielding film 6 undergoes irregular reflection by interfaces of different films under the shielding film 6, and the reflected light enters the vertical transport portion 3-1, thereby generating charges. The thus generated charges cause noise to the signal charges transferred from the vertical transfer portion 3-1, and what is worse, this noise accumulates at each time of transfer and manifests itself as smear in the image.
In summary, the convex lens formed by the conventional process poses a problem that sensitivity decreases and smear readily occurs when the pixel is reduced.
On the other hand, any attempt to reduce the curvature of the convex lens despite small pixel size results in variation in resist thickness. This in turn causes the convex lenses to vary in shape, with the result that pixel sensitivity varies in the same element or from one element to another and hence sensitivity varies in the same image.
One way to overcome this problem in the conventional CCD imaging element is to slightly increase the resist thickness for the convex lens although it somewhat causes smear and decreases sensitivity due to oblique incident rays.
Consequently, there has been a demand for a new technology which solves the above-mentioned problem. Such a new technology should be able to eliminate smear and keeps sensitivity for oblique incident rays even though the convex lens under the OCL has a small curvature for uniform sensitivity.